In various electrotechnical applications, it is often necessary to drive capacitive elements such as capacitors, for example, but also special actuators having capacitances like, in particular, piezoelectric actuators, and to precharge them prior to the actual driving. For example, capacitive control elements such as piezoelectric actuators are used as valve elements in injection systems for internal combustion engines in motor vehicles, since they are precisely controllable and allow a low-consumption injection of fuel into the corresponding combustion chamber.
A driving circuit, which also permits simple methods for charging and discharging piezoelectric actuators, is described in German Patent Application No. 10 2004 037 720 A1, for example. As a rule, the charging current occurring during the charging of piezoelectric actuators must be limited so as not to damage the semiconductor components used. Therefore, usually a pulsed charging current is obtained by switching a charging-current source on and off until the desired charging voltage is reached. After a desired charging voltage of the corresponding piezoelectric actuator has set in, it may be activated as control element in order to enable a valve opening, for example. In this context, it is desirable to be able to determine the characteristic of the charging voltage over time as precisely as possible in operation, in order to be able to carry out the fuel injection for the corresponding engine in a manner beneficial from the standpoint of efficiency.
An object of the present invention is to provide an improved method and devices for charging a capacitive element to a charging voltage.